Semiconductors, both crystalline and amorphous, are extremely important in modern day electronics and are used in the fabrication of many types of devices, e.g., photodetectors, lasers and memory circuits. Although many processing sequences are used in fabricating these and other devices, an essential step in fabricating all devices, particularly when the fabrication technique is molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), is the preparation of semiconductor surfaces that are smooth, clean and undamaged.
In many processing sequences, such surfaces are prepared by chemical etchants. The etchant typically has two or more components. A first component oxidizes and/or reduces the semiconductor constituents to form reaction products and a second component acts as the solvent for the reaction products produced by the first component. For example, silicon may be etched by a mixture consisting of HNO.sub.3 (nitric acid) and HF (hydrofluoric acid). For this combination, the oxidizing agent is HNO.sub.3 and the solvent is HF. Another etchant which was developed for germanium, but was also used with silicon, consisted of HF, HNO.sub.3, CH.sub.3 COOH and a small amount of bromine. This etchant was widely used and is referred to as CP-4. Chemical etching of silicon is surveyed in detail in Journal of the Electrochemical Society, 123, pp. 1903-1909, Dec. 19, 1976. Another commonly used semiconductor compound, GaAs, is commonly etched by a mixture of Br.sub.2 --CH.sub.3 OH (methanol) which is commonly referred to as bromine-methanol. This etchant is described in U.S. Pat. No. 3,262,825 issued on July 26, 1966. Other etchants such as NaOH--H.sub.2 O.sub.2 (sodium hydroxide-hydrogen peroxide), H.sub.2 SO.sub.4 --H.sub.2 O.sub.2 --H.sub.2 O (sulfuric acid-hydrogen peroxide-water), and NH.sub.4 OH--H.sub.2 O.sub.2 --H.sub.2 O (ammonium hydroxide-hydrogen peroxide-water) have also been used with GaAs. Chemical etching of these and other semiconductor materials is reviewed at length in RCA Review, 39, pp. 278-308, June, 1978.
The quality of semiconductor surfaces prepared by etchants is not as easily assessed as might be thought and a variety of methods has been developed to assess the effectiveness of the different chemical etchants as well as other preparatory techniques. For example, the semiconductor surface may be examined with an optical microscope, a secondary ion mass spectrometer or a scanning electron microscope. Other methods, such as Auger electron spectroscopy or LEED (low energy electron diffraction) are also used but require ultra high vacuum. Moreover, the results are often ambiguous because of oxidation and contamination resulting from exposure to the ambient atmosphere while the sample is transported to the vacuum chamber.
Another method, spectroscopic ellipsometry, has recently been brought to a high state of perfection. This technique, which may be used in the ambient atmosphere, measures the apparent dielectric function &lt;.epsilon.&gt;=&lt;.epsilon..sub.1 &gt;+i&lt;.epsilon..sub.2 &gt; of the material directly after treatment. At the wavelength of the E.sub.2 peak of the &lt;.epsilon..sub.2 &gt; spectrum it permits a very sensitive and unambiguous indication of the dielectric discontinuity between the substrate and ambient. E.sub.2 is the maximum value of &lt;.epsilon..sub.2 &gt;. The measurements yield information about the presence of residual oxides and other overlayers as well as the selvedge region and surface microstructure and bulk degradation effects. This method is described in Journal of Vacuum Science and Technology, 17, pp. 1057-1060 (1980), and relies on the fact that an overlayer with a dielectric response having a magnitude between the dielectric responses of the substrate and the ambient will impedance match the ambient to the substrate. This match reduces the amount of light reflected from the substrate. If the ellipsometric data are evaluated in a two phase model which assumes a mathematically sharp boundary between the substrate and ambient and ignores the possible presence of boundary layers, the impedance matching can be simulated only by reducing the apparent or pseudo dielectric function &lt;.epsilon.&gt; of the substrate. The quantitative relation between &lt;.epsilon.&gt; and the true substrate dielectric function .epsilon..sub.s =.epsilon..sub.s1 +i.epsilon..sub.s2 is approximately ##EQU1## where .epsilon..sub.a and .epsilon..sub.o are the dielectric functions of the ambient and a uniform overlayer of thickness, d, respectively and .lambda. is the wavelength of light. It is further assumed that d&lt;&lt;.lambda.. Also, .epsilon..sub.o and d may refer to effective averages for graded regions. Degradation of the bulk leads to a similar expression.
The wavelength corresponding to the energy of the E.sub.2 peak of the dielectric function spectrum &lt;.epsilon..sub.2 &gt;.congruent..epsilon..sub.s2 is a logical measurement choice for assessing semiconductor surfaces for at least two reasons. First, the absorption coefficient is near maximum and as a result, there is minimum light penetration into the semiconductor and maximum surface sensitivity. Second, the E.sub.2 peak itself tends to have a relatively high value due to the unique combination of chemistry and crystal structure of the substrate and if either is modified, &lt;.epsilon..sub.2 &gt; is reduced and usually substantially. As a result, all overlayers tend to look alike from the perspective of the substrate and the impedance matching argument at this wavelength is essentially universal.
While the known etching methods are perfectly adequate for many purposes, methods for producing still more abrupt discontinuities in the dielectric function, and thus cleaner and smoother surfaces, are desirable.